You are right, there is a lot about this on line. I did find two suggestions you did not mention: one suggestion was to uninstall Firefox and see if that fixed the problem. Another suggestion was to upgrade to a newer System. You are still at Sierra. Consider High Sierra or even Mojave. I have found the latter to be trouble-free on my even-older-than-yours MacBook Air.

On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.